Automatic control system with pseudoposition feedback and monitor

ABSTRACT

An automatic control system using digital techniques to drive a stepper motor for providing precisely controlled incremental output. A preselected output position triggers a synchronizing signal to ensure periodic alignment between the input signal and the corresponding output control position. Monitoring capability is provided by a memory comparison system and thereby provides a fail-operational feature. A multiplexing capability may be incorporated.

United States Patent Ferguson et al.

[54] AUTOMATIC CONTROL SYSTEM WITH PSEUDOPOSITION FEEDBACK AND MONITOR [72] Inventors: Donald C. Ferguson, Scottsdale; Frederick W. Lynch; Gerald F. Simons, both of Phoenix, all of Ariz.

[73] Assignee: Sperry Rand Corporation, Great Neck,

[22] Filed: Apr. 27, 1970 [21] Appl. No.: 31,986

Feb. 15,1972

Primary Examiner-Benjamin Dobeck Attorney-S. C. Yeaton [57] ABSTRACT An automatic control system using digital techniques to drive a stepper motor for providing precisely controlled incremental output. A preselected output position triggers a synchronizing [52] US. Cl ..3l8/565, 318/603, 318/685 signal to ensure periodic alignment between the input Signal [5 1] Gosh 23/02 and the corresponding output control position. Monitoring [58] Field of Search .318/599, 685, 603, 565 capability is provided by a memory comparison System and thereby provides a fail-operational feature. A multiplexing capability may be incorporated.

6 Claims, 3 Drawing Figures 3 TRIM GA-TE i S MOTOR 36 31 M oron C care S3 I 34 IRNATTEOGR- 1 7 "1.. c ou NTE R 33 2 e ATE Q 10 S l. S M o TOR 1h ssolfigll SHAPER A A A A THEE-HOLD f 33:2: Q

' 4 6 THRESHOLD SEOUENCER q GAT E 12 1 14 P u L s E 11 PARAMETER PULSE To ANALOG cournou. GENERATOR MEMORY ELEMENT PATENIEUFEB 15 m2 SHEET 1 OF 3 IN VE/VTORS 00mm c FERGUSON DER/ck w. LY/VCH GERALD F. S/MO/VS BY mwkznou MMJ 1 RT QM.

mma Im ATTORNEY AUTOMATIC CONTROL SYSTEM WITH PSEUDOPOSITION FEEDBACK AND MONITOR BACKGROUND OF THE INVENTION lfField of the Invention The invention relates to a control system requiring a precise incremental position change of the controlled element in proportion to the change of the indicated input controlling signal. Digital techniques are known to provide such a capability but previous implemented systems were too complex mechanically and electronically to be feasible for any but the most elaborate systems. Analog systems can provide an incremental output change but require extensive and complex mechanical implementations which prohibit the use of the system where low cost and weight are major considerations.

2. Description of the Prior Art In prior analog feedback control systems the positioning motor had to operate in conjunction with a position transducer connected to the controlled apparatus. In operation, the desired input signal was fed to each and the disparity in positionof the controlled apparatus between the existing and the desired position, as determined by the transducer, controlled the amount of movement of the positioning motor. The system was a highly complex electromechanical system requiring delicate and finite adjustments to amalgamate the mechanical and the electrical portions.

.Additionally, in analog-responsive motor systems, the motor will overshoot until an overshoot error signal is generated. Oscillation about the desired position ensues unless the further complexity of velocity control is restored to.

Prior digital control techniques, or more accurately termed, pseudo-digital control techniques, required complex analogto-digital and digital-to-analog converters. The positioning motor required an analog signal and some feedback means to the motor driving circuits from some position determination means attached to the controlled apparatus. t

The present system obviates the previously required feedback means dependent upon the position of the controlled element and embodies true digital techniques for positioning the controlled apparatus. 7

SUMMARY OF THE INVENTION An automatic displacement feedback control system is described which uses digital techniques for signal processing and to drive a servomotor of the stepper motor type by controlling the number of pulses fed to the motor. At the same time, the same number of pulsesor steps are fed to a volatile memory, the output of which provides a pseudo-feedback to close the loop, The control system provides precise and accurate control without the usual mechanical and electrical complexity of servo-driven feedback mechanisms. The monitoring system compares the steps fed to the volatile memory element to an identical memory element driver fed from the motor mechanics. This monitoring system provides a fail-operational characteristic. Multiplexing equipment may be added to the basic system where a plurality of inputs and respective controlled elements are utilized.

A primary purpose of the disclosed system is that of providing a low-cost digital feedback control system. I

Another object of the invention is to provide a monitoring capability to obtain a fail-operational capability in a digital system.

Another object of the invention is to provide a light weight, low cost control system that is highly reliable and suitable for implementation in aircraft.

Another object of the invention is to provide a control system suitable for implementation in drone aircraft.

Another object of the invention is to provide a control system operable by either remote radio control or by a position orientation system incorporated within the device to be controlled.

Another object of the invention is to provide a control system easily and inexpensively adaptable to either an analog or digital input signals.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a preferred embodiment of the basic digital automatic control system;

FIG. 2 illustrates a digital automatic control system of the type shown in FIG. I and including a fail-operational feature; and

FIG. 3 illustrates a similar system with multiplexing capability.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a signal source 1 provides an output responsive to an implemented control signal. In one application, the signal source could be a position sensor such as a gyroscope responsive to the attitude of an airplanev For the sake of simplicity of terminology and as an exemplar application, the following description will be related to an aircraft automatic flight control system. The controlled element 14 may then be considered as any one of the three primary axis control surfaces. The output of the signal source 1, whether it be an error signal from a gyroscope or some other signal, is representative of a desired or required movement of the controlled surface 14. The signal, which may be an analog signal, is passed through a shaping circuit 2 so that it conforms with the system dynamics and thence to a summing junction 4. Conventional integration means 3 may be employed at the output of the shaper 2 and the integrated input signal may be fed to the summing junction 4 to cancel out any long term or persistent error in aircraft attitude. The output of the summing junction 4 is coupled to two threshold devices 5. 6. one responsive to a positivegoing signal and the other to a negative-going signal. The threshold level of each device may be determined mathematically or empirically, dependent upon the sensitivity of the response desired. A signal of sufficient I magnitude will trigger either the positive or negative threshold devices to thereby produce an enabling signal at the respective gate 7 or 8.

Associated with gates 7 and 8 is a pulse generator 9. The repetition rate of the pulse generator 9 may be preset or it may be variable through parameter control means. As will be later described, the pulse repetition rate is proportional to the speed at which the controlled element 14 will react in response to the input signal.

When either gate 7 or 8 is enabled by the threshold devices 5, 6, pulses from .the pulse generator 9 are fed to the motor pulse sequencer 10. The motor pulse sequencer 10 may be of any configuration .whereby the pulse signals are converted to driving signals for rotating the stepper motor 12. The description of a configuration which was found satisfactory follows. The output of gates 7 and 8 feed an up/down counter of modulo three mode and has the output decoded in a decoder. The output of the decoder activates the appropriate sets of coils in the command order within the stepper motor 12 casing the rotor of the stepper motor 12 to rotate in response to the number of pulses received. The direction of rotation is determined by whether gate 7 or 8 is enabled. Attached to the stepper motor 12 is a reversible gear train 13, which in turn is attached to the controlled element 14. The characteristics of this motor make a clutch unnecessary.

Concurrently with each pulse supplied to the motor pulse sequencer 10,.a pulse is supplied to a pulse to analog memory element 11. Appropriate logic functions within the memory element 11 cause it to count up or count down in response to the pulses received. The pulses received by the memory element 11 may be considered as representative of the rotational change of the stepper motor 12 as they are the pulse commands and anassumption is therefore made that the stepper motor 12 has in fact rotated in response to the pulse command.

The output analog signal of the memory element 11 equivalent to the pulse command is fed back to the summing junction 4. The algebraic sum of the signal from the shaper 2, integrator 3 and memory element 11 then becomes the signal presented to the threshold devices 5 and 6. If it is assumed that the output of the integrator 3 is zero or negligible, then, when the value of the error signal from the memory element 11 reaches the value of the error signal from the shaper 2, the

output of the summing junction 4 is below the threshold limit or zero. The enabling signal is then turned off and the opened gate closes causing the pulse train to the motor pulse sequencer to cease. At this time, the motor position and the pulse-to-analog memory element output voltage are in coincidence. Y

The system as described is a zero-seeking system in that the ultimate position of the controlled element 14 is that of the reference or zero position. By taking advantage of this feature, a quasi-synchronizing signal can be developed periodically. If there is attached to the controlled element 14 a device which is responsive to the zero position of the controlled element 14, an output of the device may be used to clear the memory element 11 and periodically synchronize the memory element 11 to the controlled element 14. Such a device operating on the memory element 11 will then effectively remove any noise pulses that have entered the system and created an output from the memory element 11. Thus, any spurious pulses that have entered the system since the last time the controlled element 14 was at zero are erased and the system is again synchronized.

If desired, a position other than the zero position of the controlled element 14 may be used as the synchronizing position. Such a modification maybe accomplished by mechanically establishinga new position of the controlled element 14 as the point at which a synchronizing signal is emitted from the device and so recognized by the memory element 11. The memory element 11 must also be adjusted so that it contains a signal equivalent to the displacement of the controlled element 14 from the zero position. In this manner, the content of the memory element 11 will synchronize with the preselected position of the controlled element 14 each time that the controlled element 14 travels through synchronizing point and spurious signals will be deleted.

The system described above may also be used to advantage to ensure synchronization during initial startup. This may be accomplished by initiating a fixed program to the stepper motor 12 to cause it to move to the zero position. At that point the memory element 11 output will become zero and match the controlled element 14 zero position.

In any aircraft, a change of center of gravity about the pitch axis or the roll axis will have a tendency to affect the aerodynamic path of the aircraft. [n the system as described, this change can be compensated for by an appropriate movement of the controlled element 14. However, to rely on the basic system presents a major disadvantage. When the aircraft is under the control of the system, the stepper motor 12 is quite capable of holding the necessary unbalance at the controlled element 14, however, upon disengagement of the system, a severe transient can result in the aircraft attitude. To overcome this disadvantage, this system or any other aircraft control system should incorporate a trim feature.

The instant control system configuration lends itself admirably to a trim system which can utilize the simplicity aspects of the instant invention. The trim configuration is obtained by applying the output from the integrator 3 to each of two AND-gates and 31. The second input from each AND gate is obtained from one of the threshold circuits 5 and 6. One of the AND gates will then be responsive to the positive threshold and the other to the negative threshold. Under a condition where there is an output from both the integrator 3 and one of the threshold circuits, a demand for trim is present. The output of the activated AND gate enables the associated enable gate 32 or 33 permitting pulses emanating from the pulse generator 9 through counter 17 to energize the trim motor pulse sequencer 35 and drive the trim stepper motor 35 and thereby operate the respective trim tab 36. The operation of the system between the enabling gates and the trim tab is 1 similar to that described for the basic system.

A satisfactory trim system requires that the trim operation be slow so as not to c ause a destabilizing effect. Previously, such an effect has been obtained by massive and extensive gearing operations. The instant invention obviates the need for such cumbersome devices as the speed of the trim stepper motor 34 is a direct function of the pulse repetition rate of the pulse generator 9 In most cases the pulse repetition rate in the basic system is too high for satisfactory trim control. By the addition of a simple counter 17, the extant pulse repetition rate can be reduced to any satisfactory level.

A further feature available from the control system is that of an automatic control wheel steering mode. If the pilot overpowers the controlled element 14, the engage attitude will change causing the error signal from the signal course 1, Le, a gyroscope, to build up. This will enable gates 7 or 8 through thresholds 5 or 6 and pulses from the pulse generator 9 will be fed to the memory element 11 until its output to the summing junction 4 cancels the error signal. While the controlled element 14 was being overpowered by the pilot, the stepper motor 12 could not move in response to the pulses. Since the stepper motor position is physically independent of the memory element 11, the control system is zeroed and a new synchronized attitude has been established.

The basic system previously described easily lends itself to a fail-operational mode with only slight modifications, and a detailed explanation of the common elements and common operation will not be undertaken. Referring to H6. 2, attached to the stepper motor 12 by mechanical means is a mechanical pulser 17. This mechanical pulser 17 is a sector switch with appropriate contacts to generate a pulse each time that the stepper motor 12 rotates one increment. The output of the mechanical pulser l7, representing the actual controlled element movement, is summed through pulse-toanalog memory 16, at the summing junction 4 with the attitude information provided by the signal source 1. This operation is similar in concept to that described in the basic system. The zero position synchronizing feature in the basic system is herein accomplished by a pulse responsive movement through the zero position in the mechanical pulser 17. This pulse is fed toboth pulse-to-analog memory elements 11 and 16 and cleans them of spurious error causing pulses.

The monitoring feature is performed by comparing the mechanical output of the system to the motor drive inputs and is accomplished in the following manner. The output of the mechanical pulser 17 is fed through the pulse-to-analog memory 16 to obtain an analog output. This analog output establishes one input to comparator 15. The second input to comparator 15 is obtained from the pulse-to-analog memory 11 and represents the motor drive input. A direct pulse comparison rather than analog signal comparison, could be made, but phasing problems would be evident and compensatory circuitry would create additional unnecessary system complexity.

If the analog result of both pulses is not present during the sampling period, an output, through the OR-gate 18 to clamp 21, initiates application of a DC voltage to two stepper motor windings through the motor pulse sequencer l0 and locks the stepper motor 12. At the same time, the normally commanded motor pulse sequencer 10 output is quashed. If the analog result of both the pulses is present at the comparator 15 during the sampling period, an output is fed to one of the inputs of AND-gate 20.

Under normal operation, one of the operating gates, 7 or 8, will feed a pulse to monitor 19. Monitor 19 will in turn send a pulse to the second input of AND-gate 20. The AND-gate 20 will then enable gates 7 or 8. If a failure should occur in the threshold circuits or in the gates 7 and 8 such that pulses from each gate are simultaneously emitted to the motor pulse sequencer l0 and the pulse-to-analog memory 11, the same pulse will also be present at the input to monitor 19. Under that condition, the monitor 19 will recognize that an error exists and send a pulse to OR-gate 18. The OR-gate 18 will then clamp the stepper motor 12 as described for a failure indication seen by comparator 15.

In a fail-operational system the dual servos controlled by each channel are coupled through a differential gear train to provide a constant torque output upon loss of a stepper motor. To obtain constant torque under such a condition, the gear train angular velocity output is necessarily halved. The desired effect on the controlled element 14 in relation to the desired effect as measured by the input signal is destroyed. This condition may be obviated by initiating a signal from the motorclamping circuit 21 which will double the pulse repetition rate of the pulse generator 9 in the parallel or second channel. The speed of the stepper motor 12 of the good or second channel will now double causing the gear train angular velocity output to increase to its former rate.

Referring to FIG. 3, there is illustrated a modification of the basic system contained in and described in respect to FIG. I, wherein a multiplexing system is added. The inputs, position sensors la, lb, 1c, shapers 2a, 2b, 2c, integrators 3a, 3b, 3c, and summing junctions 24a, 24b, 24c represent the input signal from each of the three axes in an automatic pilot system directed to airborne vehicles. The trim features associated with the a input may be for the pitch axis and operates as previously described. In addition, the other axis may also contain similar trim features. Each of the inputs is fed as a parallel input to a first multiplexing unit 28, which may be by time, frequency, or phase division. The serial output therefrom is fed to unit 27. Unit 27 may comprise the circuit as described in respect to FIG. 1 including the threshold circuits 5, 6, gates 7, 8, pulse generator 9, and motor pulse sequencer 10. The output of unit 27 from the motor pulse sequencer is fed to a second and complementaiy DC multiplexing unit 29. Associated with both multiplex units 28 and 29, there is a timing circuit 26 of any configuration to synchronize the units.

The outputs to stepper motors 12a, 12b, 12:, memory elements Ila, 11b, 11c, gear trains 13a 13b, 13c, and controlled surfaces 14a, 14b, 14c correspond in function to the abovediscussed inputs. The memory element output associated with each stepper motor is combined with its respective input signal at summing junction a, 25b, or 25c. At the summing junction, the feedback signal modifies the respective input signal representing the desired position change of the respective controlled element.

A zero position sensing device is associated with each controlled element and upon activation will clean or realign the respective memory element. Thereby, synchronization between the actual controlled element position and the commanded controlled element position will be effected.

The trim function associated with channel a operates in the same manner as previously discussed. The output from unit 27 to AND-gates 30 and 31 is the same output shown in FIG. 1 from the threshold devices 5 and 6. Unit 32 includes the electronic trim function circuitry described in FIG. 1 and the output from unit 27 thereto is the output of the pulse generator 9 to the counter 17. As a further modification, any of the other input signals may include a trim feature.

A major advantage of the above-described system is that of reduction of required components. Only one unit 27 with its circuitry is required. The addition of the multiplex units 28 and 29 does not represent a substantial increase of components as any of many well-known simple switching circuits may be employed.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

We claim:

1. A control system for positioning a movable element in response to an analog input signal representative of a desired positional change of said movable element, comprising pulse sequencer means,

electronic circuitry means enabled by said analog input signal and driving said pulse sequencer means,

stepper motor means responsive to said pulse sequencer means,

said movable element positioned by said stepper motor means,

feedback circuit means responsive to said electronic circuitry means and providing an analog feedback signal, and

analog combining means for algebraically combining said analog feedback signal with said analog input signal,

whereby said movable element is positioned in accordance with said analog input signal.

2. The control system as described in claim I wherein said electronic circuitry means comprises a threshold circuit having both positive and negative thresholds,

a plurality of gates selectively enabled by said threshold circuit, and

a pulse generator feeding said pulse sequencer through one of said enabled gates, whereby said stepper motor means is caused to rotate.

3. The control system as described in claim 2 including means for segregating a long term component from said analog input signal, and

means for combining said long term component with said analog input signal and said analog feedback signal.

4. A control system for positioning a movable element in response to an input signal representative of a desired positional change of said movable element, comprising pulse sequencer means,

electronic circuitry means enabled by said input signal and driving said pulse sequencer means,

stepper motor means responsive to said pulse sequencer means,

said movable element positioned by said stepper motor means,

feedback circuit means responsive to said electronic circuitry means and providing a feedback signal,

means for algebraically combining said feedback signal with said input signal,

whereby said movable element is with said input signal, and

synchronizing means comprising a pickoff coupled to said movable element which provides a synchronizing signal to said feedback circuit means, and means responsive to said synchronizing signal for periodically synchronizing said feedback signal with the movable element position, whereby coincidence between the desired and the actual position of said movable element is effected.

5. The control system as claimed in claim 4 wherein said feedback circuit means comprises a pulse to analog memory element whereby said synchronizing signal resets said memory element at a value equivalent to the position of said movable element thereby providing said periodic synchronizing.

6. A control system for positioning a movable element in response to an input signal representative of a desired positional change of said movable element, comprising pulse sequencer means,

electronic circuitry means enabled by said input signal and driving said pulse sequencer means,

stepper motor means responsive to said pulse sequencer means,

said movable element positioned by said stepper motor means,

feedback circuit means responsive to said electronic circuitry and providing a feedback signal,

means for algebraically combining said feedback signal with said input signal,

whereby said movable element is positioned in accordance with said input signal,

said electronic circuitry means comprising a threshold circuit having both positive and negative thresholds, a plurality of gates selectively enabled by said threshold circuit and a pulse generator feeding said pulse sequencer through one of said enabled gates, whereby said stepper motor means is caused to rotate, and

positioned in accordance fed by said positive and negative threshold devices respectively, and a counted fed by said pulse generator. said AND gates selectively enabling said counter to activate said trim motor pulse sequencer whereby said trim element is controlled by said long term signal. 

1. A control system for positioning a movable element in response to an analog input signal representative of a desired positional change of said movable element, comprising pulse sequencer means, electronic circuitry means enabled by said analog input signal and driving said pulse sequencer means, stepper motor means responsive to said pulse sequencer means, said movable element positioned by said stepper motor means, feedback circuit means responsive to said electronic circuitry means and providing an analog feedback signal, and analog combining means for algebraically combining said analog feedback signal with said analog input signal, whereby said movable element is positioned in accordance with said analog input signal.
 2. The control system as described in claim 1 wherein said electronic circuitry means comprises a threshold circuit having both positive and negative thresholds, a plurality of gates selectively enabled by said threshold circuit, and a pulse generator feeding said pulse sequencer through one of said enabled gates, whereby said stepper motor means is caused to rotate.
 3. The control system as described in claim 2 including means for segregating a long term component from said analog input signal, and means for combining said long term component with said analog input signal and said analog feedback signal.
 4. A control system for positioning a movable element in response to an input signal representative of a desired positional change of said movable element, comprising pulse sequencer means, electronic circuitry means enabled by said input signal and driving said pulse sequencer means, stepper motor means responsive to said pulse sequencer means, said movable element positioned by said stepper motor means, feedback circuit means responsive to said electronic circuitry means and providing a feedback signal, means for algebraically combining said feedback signal with said input signal, whereby said movable element is positioned in accordance with said input signal, and synchronizing means comprising a pickoff coupled to said movable element which provides a synchronizing signal to said feedback circuit means, and means responsive to said synchronizing signal for periodically synchronizing said feedback signal with the movable element position, wherebY coincidence between the desired and the actual position of said movable element is effected.
 5. The control system as claimed in claim 4 wherein said feedback circuit means comprises a pulse to analog memory element whereby said synchronizing signal resets said memory element at a value equivalent to the position of said movable element thereby providing said periodic synchronizing.
 6. A control system for positioning a movable element in response to an input signal representative of a desired positional change of said movable element, comprising pulse sequencer means, electronic circuitry means enabled by said input signal and driving said pulse sequencer means, stepper motor means responsive to said pulse sequencer means, said movable element positioned by said stepper motor means, feedback circuit means responsive to said electronic circuitry and providing a feedback signal, means for algebraically combining said feedback signal with said input signal, whereby said movable element is positioned in accordance with said input signal, said electronic circuitry means comprising a threshold circuit having both positive and negative thresholds, a plurality of gates selectively enabled by said threshold circuit and a pulse generator feeding said pulse sequencer through one of said enabled gates, whereby said stepper motor means is caused to rotate, and further including a trim system comprising trim stepper motor means for controlling a trim element, a trim motor pulse sequencer for driving said trim stepper motor, means for segregating a long term component of said input signal, two AND gates, each having one of its inputs fed by said long term signal and the second of its inputs fed by said positive and negative threshold devices respectively, and a counted fed by said pulse generator, said AND gates selectively enabling said counter to activate said trim motor pulse sequencer whereby said trim element is controlled by said long term signal. 